1. Field of the Invention
A certain aspect of the present invention relates to an electronic volume circuit.
2. Description of the Related Art
Patent Application Publication No. 7-254830 discloses an electronic volume circuit including an operational amplifier. Although the electronic volume circuit disclosed in Patent Application Publication No. 7-254830 does not have a multi-channel mixing function, recent electronic volume circuits (such as a line-out amplifier and a headphone amplifier) are expected to have a high signal-to-noise ratio and low distortion characteristic as well as to include various functions such as a multi-channel mixing function, a multi-step electronic volume function, and a function to generate an output signal that is based on ground potential (this in turn makes it possible to reduce the number of external parts).
FIG. 4 is a circuit diagram of a related-art electronic volume circuit. The electronic volume circuit of FIG. 4 includes a controller 50; input terminals T1 and T2 for inputting input signals Si1 and Si2 that are audio signals; a variable capacitor 53 used as an input capacitor; a switch 51 connected between the input terminal T1 and the variable capacitor 53; a variable capacitor 54 used as an input capacitor; a switch 52 connected between the input terminal T2 and the variable capacitor 54; an operational amplifier 57; a capacitor 55 and a resistor 56 connected in parallel between the inverting input terminal and the output terminal of the operational amplifier 57; an output terminal T3 for outputting an output signal Sout from the operational amplifier 57; and a load resistor 9. One electrode of the variable capacitor 53 is connected via the switch 51 to the input terminal T1 and the other electrode of the variable capacitor 53 is connected to the inverting input terminal of the operational amplifier 57. Similarly, one electrode of the variable capacitor 54 is connected via the switch 52 to the input terminal T2 and the other electrode of the variable capacitor 54 is connected to the inverting input terminal of the operational amplifier 57. The non-inverting input terminal of the operational amplifier 57 is grounded; the inverting input terminal and the non-inverting input terminal are virtually short-circuited; and the output terminal is grounded via the load resistor 9. The capacitor 55 is a feedback capacitor and the resistor 56 is a feedback resistor.
As shown in FIG. 4, the controller 50 outputs pass control signals to the switches 51 and 52 to turn on at least one of the switches 51 and 52. The controller 50 also outputs volume control signals to the variable capacitors 53 and 54 to set the capacitance values of the variable capacitors 53 and 54 at specific values. Thus, the electronic volume circuit of FIG. 4 is configured to mix and variably amplify or attenuate the input signals Si1 and Si2 that are based on a predetermined reference voltage level and thereby to output the output signal Sout that is based on ground potential.
The resistor 56 and the capacitor 55 constitute a high-pass filter; and the variable capacitors 53 and 54 and the switches 51 and 52 having on-resistances constitute a low-pass filter. The cut-off frequency of the high-pass filter is set at a value such that the output signal Sout with a predetermined level is output at 20 Hz that is the lower limit of the audio signal band. Here, depending on the resistance value of the resistor 56, the signal-to-noise ratio may become low because of the thermal noise generated by the resistor 56. Therefore, for example, the resistance value of the resistor 56 is set at several hundred MΩ and the capacitance value of the capacitor 55 is set at several tens pF. The capacitance values of the variable capacitors 53 and 54 are determined based on the capacitance value of the capacitor 55 and the gain required for the electronic volume circuit.
Since the on-resistances of the switches 51 and 52 are comparatively low, the pole of the low-pass filter is sufficiently higher than 20 kHz that is the upper limit of the audio signal band. When it is necessary to set the pole of the low-pass filter in a lower band, resistors are added before and after the switches 51 and 52 in the electronic volume circuit.
With the configuration of the related-art electronic volume circuit shown in FIG. 4, the sizes of the variable capacitors 53 and 54 necessary when amplifying the input signals Si1 and Si2 are greater than those necessary when attenuating the input signals Si1 and Si2. Also with the configuration of the related-art electronic volume circuit, increasing the number of input signals makes it necessary to increase the number of variable capacitors according to the number of input signals and thereby makes it necessary to increase the die size of a semiconductor integrated circuit including the electronic volume circuit.